Pololu 19:1 Metal Gearmotor 37Dx52L mm with 64 CPR Encoder is a powerful 12V brushed DC motor with a 18.75:1 metal gearbox and an integrated quadrature encoder that provides a resolution of 64 counts per revolution of the motor shaft, which corresponds to 1200 counts per revolution of the gearbox’s output shaft. These units have a 15.4mm (0.61") long, 6 mm-diameter D-shaped output shaft.
Pololu 19:1 Metal Gearmotor 37Dx52L mm with 64 CPR Encoder is intended for use at 12 V, though in general, these kinds of motors can run at voltages above and below the nominal voltage (they can begin rotating at voltages as low as 1 V). Lower voltages might not be practical, and higher voltages could start negatively affecting the life of the motor.
Note: This motor will run at 6 V but is intended for operation at 12 V.